planispirale

Planispiral is a term used to describe a shell or organism whose coiling lies in a single geometric plane. In planispiral coiling, the whorls grow around a central axis but the coil as a whole lies flat, producing a disc-like or flat shell. The axis of coiling is approximately perpendicular to the plane of the coil.

Morphology and characteristics. The defining feature is that successive whorls are arranged in the same plane, giving a nearly circular footprint. The aperture orientation and the exact shape can vary, but the shell tends to be thin and flattened compared with more tall, conical spirals. This contrasts with conispiral or helically coiled shells, where the coil rises above the previous whorl to form a spire.

Occurrence and examples. Planispiral coiling is found in diverse groups. The best-known living examples are freshwater snails in the family Planorbidae, commonly called ram’s horn snails, whose shells are flattened and coiled in one plane. In the fossil record, planispiral coiling is widespread among ammonoids and nautiloids, as well as certain gastropod lineages. The form may reflect developmental constraints or ecological adaptations, such as a compact body plan or, in aquatic species, particular hydrodynamic or buoyancy considerations.

Significance. Planispirality is a notable character in paleontology and malacology, aiding identification and classification of fossil shells and providing insight into the evolution of shell coiling patterns across different groups.